Hashirama. Tajuu Mokuton Bunshin should give both of them a hard time, and give Hashirama an opening to knock Gaara out. About Jinton, I think Hashirama has been around long enough to know what it is and how to deal with, plus I don't see how they both Gaara and Oonoki can break free from Bringer of Darkness. The biggest threat to Hashirama in this fight would be Oonoki's Jinton.
Tajuu Mokuton Bunshin would be helpful if he had more than Mokuton at his disposal. Madara only poses a problem with it because of the EMS+Wood+Rinnegan combo.
If you can tell me a way to deal with Jinton that Hashirama can use then I'll concede that Hashirama can come up with a counter to it. But wood is useless against it.
Gaara has sand sensing so he would still be able to attack Hashirama without being able to see him.
Let's not forget Gaara's sand is faster than Amaterasu (Raikage saw, realized a threat, and dodges Amaterasu, Gaara stopped Raikage mid-attack, therefore Gaara's sand is faster than Amaterasu).
Yes Hashirama can create a lot with those trees but we saw Jinton counter the poisonous forest and we know Gaara has attacks of that magnitude to counter a regular forest.